1. Field of the Invention
The invention related generally to the field of electronic displays and more particularly to Color Liquid Crystal Displays (LCDs).
2. Description of Related Art
The LCD industry is facing a major challenge in trying to reduce the acknowledged variation in color performance from panel to panel within the same manufacturer as well as between manufacturers of the same panel. The reduction in the panel to panel variation has long been desired from manufacturers, integrators, software developers and even end users. For example, Microsoft recently issued a specification for color consistency outlined in the Windows Color Quality Specifications for Liquid Crystal Display OEMs (hereinafter referred to as Windows VISTA specification) in which Delta E measurement criteria is specified based on the IEC 61966-2-1 standard for sRGB. This technique selects a certain number of colors in the sRGB color space, gives their color coordinates (R,G,B) and compares the measured color's chromaticity and luminance to a reference. The color's error from the reference color is referred to as the Delta E for that color patch. Microsoft's specification requires that the panel's measured values are below an average value and a maximum value for a specified set of colors. Specifically, the display luminance level must be greater than or equal to 75 cd/m2 and the Delta E is required to meet the following requirements:    (a) Average 1994 Delta E less than or equal to 20 for IEC 61966-4 (section 11 for Inter-Channel Dependency) set of 32 colors.    (b) Stand alone or desktop LCDs:
Average 1994 Delta E less than or equal to 10 for desktop set of common colors.
Maximum 1994 Delta E less than or equal to 15 for desktop set of common set of colors.    (c) Integrated or notebook LCDs:
Average 1994 Delta E less than or equal to 10 for notebook set of common set of colors.
Maximum 1994 Delta E less than or equal to 15 for notebook set of common set of colors.
The Delta E calculation is weighted more heavily on changes in luminance from the reference than chromaticity, which increases the effect of changing gamma on Delta E values. The Delta E criteria of the Windows VISTA specification are much more significant than other attempts to specify the color performance of the panel, since it actually compares the measured color to a specified standard. It should be noted that the more traditional specification of gamma 2.2+/10% will result in failures in meeting the Delta E specification of the above referenced specification. It can be shown that using the current manufacturing processes and for the expected variation in gamma from panel to panel, it will be next to impossible to guarantee that all panels meet the Microsoft Delta E specification.
It can be assumed that once the panel manufacturers begin delivering to this specification for the notebook and monitor markets, that the television manufacturers will also demand the same level of consistency. The problem for the panel manufacturers is that guaranteeing some level of color performance will require additional testing and processing in the panel assembly lines, which could negatively impact cycle time, costs and yields. The primary causes of panel to panel color variation are: Gamma variation, color filter consistency and backlight color temperature. It is desirable to eliminate or compensate for the gamma variation, and hence eliminate the largest cause for panel to panel color variation.